The present invention relates to MicroElectroMechanical Systems (MEMS). More particularly, the present invention pertains to frequency selective MEMS devices.
Currently, there is an interest in increasing the degree of integration of electronics. Integration has proceeded steadily over the last few decades and achieved remarkable reduction in the physical size occupied by electronic circuits. Semiconductor lithography has enabled circuits with millions of transistors to be constructed on a single silicon die. Nonetheless, certain components are difficult to integrate.
For example, inductors are difficult to integrate. Although, certain spiral shaped designs for integrated circuits have been proposed, owing to their inherent resistive losses, these spiral inductors are ill suited for producing high Q resonators which are needed to generate stable frequency signal sources.
One important component that is used to generate stable frequencies in a variety of electronic apparatus including sequential logic (e.g., microprocessors) and wireless communication transceivers is the quartz crystal resonator. The quartz crystal resonator in its usual form is a bulky discrete component.
Micro ElectroMechanical System (MEMS) based resonators have been proposed as alternatives to quartz resonators for use as frequency selective components at RF frequencies. One type of MEMS resonator that has been proposed comprises a suspended beam of semiconductor material that is shaped and sized to resonate at a selected frequency chosen in view of a desired electrical frequency response. The MEMS resonator serves as a frequency selective component in a circuit. According to one design the MEMS resonator is driven by a drive electrode that extends below the suspended beam. Electric force interaction between the suspended beam and the drive electrode induces the suspended beam to vibrate.
In certain proposed MEMS resonators a main beam that resonates in a flexural mode, is supported by a number of support beams that meet the main beam at right angles, and attach to the main beam at node points of the flexural mode. Each support beam is dimensioned to resonate in a torsional mode that has an anti-node at an end of the support beam that attaches to the main beam, and a node at opposite ends that are anchored to a base on which the resonator is fabricated. The support beams dimensioned in this manner serve to isolate the main beam from the base, and reduce the amount of acoustic energy leaked from the main beam through the anchors into the base. The length of the support beams required to achieve a xcfx80/2 (xcex/4) phase length so as to procure a node at the anchor end and an anti-node at the main beam end greatly increases the overall area required to accommodate the resonator.
In the MEMS resonators described above a bias voltage and a oscillatory voltage component is applied between the main beam and an underlying base. These voltages serve to establish an oscillatory electric force between the main beam and the base that drives main beam into resonance. The vibrations of the main beam serve to reinforce oscillatory voltage components that correspond in frequency to the frequency of vibration of the main beam. The degree of intercoupling between the oscillatory voltage component, and the vibration of the beam depends on the magnitude of the bias voltage. Ordinarily the magnitude bias voltage is limited to being less than a magnitude that will cause the main beam to be pulled down against tension in the support beams until it is a position to mechanically interfere with the base. It would be desirable to provide support beams of increased tensile stiffness so as to allow greater bias voltages to be used.
Although a MEMS resonator occupies very little space compared to an external discrete component it does take up substantial space compared to electrical components found on integrated circuits. A single MEMS resonator can take up space on a semiconductor die that could have been used for hundreds of transistors. In some applications it would be advantageous to be able to reduce the die area occupied by a MEMS resonator.
During the past decade there has been an increased interest in the semiconductor industry in the use of Silicon-On-Insulator (SOI) wafers. SOI wafers include a silicon substrate, a silicon di-oxide layer on the silicon substrate, and a single crystal silicon layer on the silicon di-oxide layer. SOI wafers afford a number of advantages in terms of the electrical properties of circuits built using them, including reduced voltage requirements, and power consumption for a given clock speed.
In a previously filed patent application entitled xe2x80x9cMEMS RESONATORS AND METHODS FOR MANUFACTURING MEMS RESONATORSxe2x80x9d Ser. No. 09/828,431, filed on Apr. 9, 2001 and assigned to the assignee of the present invention, MEMS resonators fabricated on SOI wafers are disclosed. In the disclosed SOI MEMS resonators, a flexural mode resonant beam and a number of support beams that attach to the flexural mode resonant beam at node points are etched from the single crystal silicon layer. A portion of the silicon di-oxide layer in an area underneath the flexural mode resonant beam, and the support beams is removed by an isotropic etch to allow for free movement of the flexural mode resonant beam and the support beams. It is desirable to minimize the area occupied by such SOI MEMS resonators and at the same time minimize the leakage of acoustic energy from the SOI MEMS resonators.
Application Ser. No. 09/828,431 filed Apr. 9, 2001 is hereby incorporated herein by reference.
What is needed is a MEMS resonator that occupies a relatively small amount of die space while at the same time exhibiting a high resonant quality factor.